CSET Practice Test Subtest II Science

The Digestive System

The digestive system is a series of hollow organs joined
 in a long, twisting tube from the mouth to the anus.
 Inside this tube is a lining called the mucosa. In the
 mouth, stomach, and small intestine, the mucosa
 contains tiny glands that produce juices to help
 digest food. 

There are also two solid digestive organs, the liver 
and the pancreas, which produce juices that reach
 the intestine through small tubes. In addition,
 parts of other organ systems (for instance, nerves
 and blood) play a major role in the digestive system. 

Why is Digestion Important?
When we eat such things as bread, meat, and
vegetables, they are not in a form that the body
can use as nourishment. Our food and drink must
be changed into smaller molecules of nutrients
before they can be absorbed into the blood and
carried to cells throughout the body. Digestion is
the process by which food and drink are broken
down into their smallest parts so that the body
can use them to build and nourish cells and to 
provide energy.

How is Food Digested?
Digestion involves the mixing of food, its movement
 through the digestive tract, and chemical breakdown 
of the large molecules of food into smaller molecules. 
Digestion begins in the mouth, when we chew and 
swallow, and is completed in the small intestine. The 
chemical process varies somewhat for different kinds 
of food.

Movement of Food Through the System
The large, hollow organs of the digestive system 
contain muscle that enables their walls to move. The 
movement of organ walls can propel food and liquid 
and also can mix the contents within each organ. 
Typical movement of the esophagus, stomach, and 
intestine is called peristalsis. The action of peristalsis 
looks like an ocean wave moving through the muscle. 
The muscle of the organ produces a narrowing and 
then propels the narrowed portion slowly down the 
length of the organ. These waves of narrowing push 
the food and fluid in front of them through each hollow 
organ. 

The first major muscle movement occurs when food or 
liquid is swallowed. Although we are able to start 
swallowing by choice, once the swallow begins, it 
becomes involuntary and proceeds under the control 
of the nerves. 

The esophagus is the organ into which the swallowed 
food is pushed. It connects the throat above with the 
stomach below. At the junction of the esophagus and 
stomach, there is a ringlike valve closing the passage 
between the two organs. However, as the food 
approaches the closed ring, the surrounding muscles 
relax and allow the food to pass.

The food then enters the stomach, which has three 
mechanical tasks to do. First, the stomach must store 
the swallowed food and liquid. This requires the muscle 
of the upper part of the stomach to relax and accept 
large volumes of swallowed material. The second job is 
to mix up the food, liquid, and digestive juice produced 
by the stomach. The lower part of the stomach mixes 
these materials by its muscle action. The third task of 
the stomach is to empty its contents slowly into the 
small intestine. 

Several factors affect emptying of the stomach, 
including the nature of the food (mainly its fat and 
protein content) and the degree of muscle action of 
the emptying stomach and the next organ to receive 
the stomach contents (the small intestine). As the 
food is digested in the small intestine and dissolved 
into the juices from the pancreas, liver, and intestine, 
the contents of the intestine are mixed and pushed 
forward to allow further digestion. 

Finally, all of the digested nutrients are absorbed 
through the intestinal walls. The waste products of 
this process include undigested parts of the food, 
known as fiber, and older cells that have been shed 
from the mucosa. These materials are propelled into 
the colon, where they remain, usually for a day or two, 
until the feces are expelled by a bowel movement.

Production of Digestive Juices
The glands that act first are in the mouth--the 
salivary glands. Saliva produced by these glands 
contains an enzyme that begins to digest the starch 
from food into smaller molecules. 

The next set of digestive glands is in the stomach 
lining. They produce stomach acid and an enzyme 
that digests protein. One of the unsolved puzzles of 
the digestive system is why the acid juice of the 
stomach does not dissolve the tissue of the stomach 
itself. In most people, the stomach mucosa is able to 
resist the juice, although food and other tissues of the 
body cannot. 

After the stomach empties the food and its juice into 
the small intestine, the juices of two other digestive 
organs mix with the food to continue the process of 
digestion. One of these organs is the pancreas. It 
produces a juice that contains a wide array of 
enzymes to break down the carbohydrates, fat, and 
protein in our food. Other enzymes that are active in 
the process come from glands in the wall of the 
intestine or even a part of that wall. 

The liver produces yet another digestive juice--bile. 
The bile is stored between meals in the gallbladder. 
At mealtime, it is squeezed out of the gallbladder into 
the bile ducts to reach the intestine and mix with the 
fat in our food. The bile acids dissolve the fat into the 
watery contents of the intestine, much like detergents 
that dissolve grease from a frying pan. After the fat is 
dissolved, it is digested by enzymes from the pancreas 
and the lining of the intestine.

Absorption and Transport of Nutrients
Digested molecules of food, as well as water and 
minerals from the diet, are absorbed from the cavity 
of the upper small intestine. The absorbed materials 
cross the mucosa into the blood, mainly, and are 
carried off in the bloodstream to other parts of the 
body for storage or further chemical change. As noted 
above, this part of the process varies with different 
types of nutrients. 

Carbohydrates: An average American adult eats about 
half a pound of carbohydrate each day. Some of our 
most common foods contain mostly carbohydrates. 
Examples are bread, potatoes, pastries, candy, rice, 
spaghetti, fruits, and vegetables. Many of these foods 
contain both starch, which can be digested, and fiber, 
which the body cannot digest. 

The digestible carbohydrates are broken into simpler 
molecules by enzymes in the saliva, in juice produced 
by the pancreas, and in the lining of the small intestine. 
Starch is digested in two steps: First, an enzyme in the 
saliva and pancreatic juice breaks the starch into 
molecules called maltose; then an enzyme in the lining 
of the small intestine (maltase) splits the maltose into 
glucose molecules that can be absorbed into the blood. 
Glucose is carried through the bloodstream to the liver, 
where it is stored or used to provide energy for the work 
of the body. 

Table sugar is another carbohydrate that must be 
digested to be useful. An enzyme in the lining of the 
small intestine digests table sugar into glucose and 
fructose, each of which can be absorbed from the intestinal 
cavity into the blood. Milk contains yet another type of 
sugar, lactose, which is changed into absorbable molecules 
by an enzyme called lactase, also found in the intestinal 
lining. 

Protein: Foods such as meat, eggs, and beans consist 
of giant molecules of protein that must be digested by 
enzymes before they can be used to build and repair 
body tissues. An enzyme in the juice of the stomach 
starts the digestion of swallowed protein. Further 
digestion of the protein is completed in the small 
intestine. Here, several enzymes from the pancreatic 
juice and the lining of the intestine carry out the 
breakdown of huge protein molecules into small 
molecules called amino acids. These small molecules 
can be absorbed from the hollow of the small intestine 
into the blood and then be carried to all parts of the 
body to build the walls and other parts of cells. 

Fats: Fat molecules are a rich source of energy for 
the body. The first step in digestion of a fat such as 
butter is to dissolve it into the watery content of the 
intestinal cavity. The bile acids produced by the liver 
act as natural detergents to dissolve fat in water and 
allow the enzymes to break the large fat molecules 
into smaller molecules, some of which are fatty acids 
and cholesterol. The bile acids combine with the fatty 
acids and cholesterol and help these molecules to 
move into the cells of the mucosa. In these cells the 
small molecules are formed back into large molecules, 
most of which pass into vessels (called lymphatics) 
near the intestine. These small vessels carry the 
reformed fat to the veins of the chest, and the blood 
carries the fat to storage depots in different parts of 
the body. 

Vitamins: Another vital part of our food that is 
absorbed from the small intestine is the class of 
chemicals we call vitamins. There are two different 
types of vitamins, classified by the fluid in which 
they can be dissolved: water-soluble vitamins (all 
the B vitamins and vitamin C) and fat-soluble 
vitamins (vitamins A, D, and K). 

Water and Salt: Most of the material absorbed from 
the cavity of the small intestine is water in which 
salt is dissolved. The salt and water come from the 
food and liquid we swallow and the juices secreted 
by the many digestive glands. In a healthy adult, 
more than a gallon of water containing over an 
ounce of salt is absorbed from the intestine every 
24 hours.

How is the Digestive Process Controlled?
Hormone Regulators
A fascinating feature of the digestive system is that 
it contains its own regulators. The major hormones 
that control the functions of the digestive system 
are produced and released by cells in the mucosa of 
the stomach and small intestine. These hormones 
are released into the blood of the digestive tract, 
travel back to the heart and through the arteries, 
and return to the digestive system, where they 
stimulate digestive juices and cause organ movement. 
The hormones that control digestion are gastrin, 
secretin, and cholecystokinin (CCK): Gastrin causes 
the stomach to produce an acid for dissolving and 
digesting some foods. It is also necessary for the 
normal growth of the lining of the stomach, small 
intestine, and colon. 

Secretin causes the pancreas to send out a 
digestive juice that is rich in bicarbonate. It stimulates 
the stomach to produce pepsin, an enzyme that 
digests protein, and it also stimulates the liver to 
produce bile. 

CCK causes the pancreas to grow and to produce 
the enzymes of pancreatic juice, and it causes the 
gallbladder to empty. 

Nerve Regulators
Two types of nerves help to control the action of 
the digestive system. Extrinsic (outside) nerves 
come to the digestive organs from the unconscious 
part of the brain or from the spinal cord. They 
release a chemical called acetylcholine and another 
called adrenaline. Acetylcholine causes the muscle 
of the digestive organs to squeeze with more force 
and increase the "push" of food and juice through 
the digestive tract. Acetylcholine also causes the 
stomach and pancreas to produce more digestive 
juice. Adrenaline relaxes the muscle of the stomach 
and intestine and decreases the flow of blood to 
these organs. 
Even more important, though, are the intrinsic (inside) 
nerves, which make up a very dense network embedded 
in the walls of the esophagus, stomach, small intestine, 
and colon. The intrinsic nerves are triggered to act 
when the walls of the hollow organs are stretched by 
food. They release many different substances that 
speed up or delay the movement of food and the 
production of juices by the digestive organs.

Energy

The conservation of energy is a fundamental concept 
of physics along with the conservation of mass and 
the conservation of momentum. Within some problem 
domain, the amount of energy remains constant and 
energy is neither created nor destroyed. Energy can 
be converted from one form to another (potential 
energy can be converted to kinetic energy) but the 
total energy within the domain remains fixed. 

Thermodynamics is a branch of physics which deals 
with the energy and work of a system. It was born in 
the 19th century as scientists were first discovering 
how to build and operate steam engines.

There are three principal laws of thermodynamics:

The zeroth law of thermodynamics begins with a 
simple definition of thermodynamic equilibrium . It is 
observed that some property of an object, like the 
pressure in a volume of gas, the length of a metal rod, 
or the electrical conductivity of a wire, can change 
when the object is heated or cooled. If two of these 
objects are brought into physical contact there is 
initially a change in the property of both objects. But, 
eventually, the change in property stops and the 
objects are said to be in thermal (thermodynamic) 
equilibrium. Thermodynamic equilibrium leads to the 
large scale definition of temperature. When two 
objects are in thermal equilibrium they are said to 
have the same temperature. During the process of 
reaching thermal equilibrium, heat, which is a form of 
energy, is transferred between the objects. The 
details of the process of reaching thermal equilibrium 
are described in the first and second laws of 
thermodynamics.

The first law of thermodynamics relates the various 
forms of energy in a system (kinetic and potential) to 
the work which a system can perform and to the 
transfer of heat. This law is sometimes taken as the 
definition of internal energy, and introduces an 
additional state variable, enthalpy.

The internal energy is just a form of energy like the 
potential energy of an object at some height above 
the earth, or the kinetic energy of an object in motion. 
In the same way that potential energy can be 
converted to kinetic energy while conserving the total 
energy of the system, the internal energy of a 
thermodynamic system can be converted to either 
kinetic or potential energy. Like potential energy, the 
internal energy can be stored in the system. Notice, 
however, that heat and work can not be stored or 
conserved independently since they depend on the 
process. The first law of thermodynamics allows for 
many possible states of a system to exist, but only 
certain states are found to exist in nature. The second 
law of thermodynamics helps to explain this observation. 

This leads to the second law of thermodynamics and 
the definition of another state variable called entropy. 
The second law stipulates that the total entropy of a 
system plus its environment can not decrease; it can 
remain constant for a reversible process but must 
always increase for an irreversible process.